The present invention relates generally to a pulsed imaging, non-facet tracked, pulse width modulation Raster Output Scan (ROS) system for creating tri-level exposures at a recording medium such as images at a photosensitive surface, and, more specifically, for asymmetric spatial filtering in the ROS system.
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The electrostatic charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a bi-level latent charge pattern on the imaging surface where the high charge regions correspond to the areas not exposed by radiation. One level of this charge pattern is made visible by developing it with toner. The toner is generally a colored powder that adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface, or is transferred to a receiving substrate such as plain paper, to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, upon exposure, three charge levels are produced on the charge-retentive surface. The highly charged (i.e. unexposed) areas are developed with toner of one color, and the area more fully discharged is also developed, but with a toner of a different color. Thus, the charge retentive surface contains three exposure levels; zero exposure, intermediate exposure, and full exposure, which correspond to three charge levels. These three levels can be developed to print, for example, black, white, and a single color.
FIG. 1 is a schematic drawing of a prior art tri-level printing system. As shown, the system utilizes a charge retentive member in the form of a photoconductive belt 10, consisting of a photoconductive surface on an electrically conductive, light-transmissive substrate mounted for movement past a charge station A, an exposure station B, developer station C, transfer station D, and cleaning station F. Belt 10 moves in the direction of arrow 16 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about a plurality of rollers 18, 20 and 22, the former of which can be used as a drive roller, and the latter of which can be used to provide suitable tensioning of the photoreceptor belt 10. Motor 23 rotates roller 18 to advance belt 10 in the direction of arrow 16. Roller 18 is coupled by motor 23 by suitable means such as a belt drive.
As can be seen by further reference to FIG. 2, initially successive portions of belt 10 pass through charging station A, where a corona discharge device such as a scorotron, corotron, or dicorotron, indicated generally by the reference numeral 24, charges the belt 10 to a selectively high uniform positive or negative potential, V.sub.0. Any suitable control circuit, as well known in the art, may be employed for controlling the corona discharge device 24.
Next, the charged portions of the photoreceptor surface are advanced through exposure station B. At exposure station B, the uniformly charged surface of belt 10 is exposed by a tri-level raster output scanner (ROS) unit 25, which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. This scan results in three separate discharge regions on the photoreceptor, each region exposed at one of three possible levels: (1) zero exposure which results in a voltage equal to V.sub.ddp and will be developed using charged-area-development (CAD); (2) full exposure, which results in a low voltage level V.sub.C and is developed using discharged-area-development (DAD); and (3) intermediate exposure, which yields an intermediate voltage level V.sub.W and does not develop and yields a white region on the print. These voltage levels are shown schematically in FIG. 2. Some typical voltage levels are as follows.
The photoreceptor, which is initially charged to a voltage V.sub.0, undergoes dark decay to a level V.sub.ddp (V.sub.CAD) equal to about -900 volts. When exposed at the exposure station B, the photoreceptor is discharged to V.sub.c, (V.sub.DAD) equal to about -100 volts in the highlight (i.e. color other than black) color portions of the image. The photoreceptor is also discharged to V.sub.w (V.sub.white) equal to -500 volts imagewise in the background (i.e. white), image areas and in the inter-document area. Thus the image exposure is at three levels; zero exposure (i.e. black), intermediate exposure (white) and full exposure (i.e. color). After passing through the exposure station, the photoreceptor contains highly charged areas and fully discharged areas which correspond to CAD and DAD color latent images, and also contains an intermediate level charged area that is not developed.
At development station C, a development system, indicated generally by the reference numeral 30, advances developer materials into contact with the CAD and DAD electrostatic latent images. The development system 30 comprises first and second developer housings 32 and 34. The developer housing 32 contains a pair of magnetic brush rollers 35 and 36. The rollers advance developer material 40 into contact with the photoreceptor for developing the charged-area regions (V.sub.CAD). The developer material 40, by way of example, contains positively charged black toner. Electrical biasing is accomplished via power supply 41, electrically connected to developer apparatus 32. A suitable DC bias, V.sub.bb, of approximately -600 volts is applied to the rollers 35 and 36 via the power supply 41.
The developer housing 34 contains a pair of magnetic rolls 37 and 38. The rollers advance developer material 42 into contact with the photoreceptor for developing the discharged-area regions (V.sub.DAD). The developer material 42, by way of example, contains negatively charged red toner. Appropriate electrical biasing is accomplished via power supply 43 electrically connected to developer apparatus 34. A suitable DC bias, V.sub.cb, of approximately -400 volts is applied to the rollers 37 and 38 via the bias power supply 43.
Because the composite image developed on the photoreceptor consists of both positive and negative toner, a positive pre-transfer corona discharge member (not shown) is provided to condition the toner for effective transfer to a substrate, using positive corona discharge. The pre-transfer corona discharge member is preferably an AC corona device, biased with a DC voltage to operate in a field sensitive mode, to perform tri-level xerography pre-transfer charging in a way that selectively adds more charge (or at least comparable charge) to the region of the composite tri-level image that must have its polarity reversed. This charge discrimination is enhanced by discharging the photoreceptor carrying the composite developed latent image with light before the pre-transfer charging this minimizes the tendency to overcharge portions of the image which are already at the correct polarity.
Referring again to FIG. 1, a sheet of support material 58 is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by conventional sheet feeding apparatus, not shown. Preferably, the sheet feeding apparatus includes a feed roll contacting the upper most sheet of a stack of copy sheets. Feed rolls rotate to advance the uppermost sheet from the stack into a chute, which directs the advancing sheet of support material into contact with the surface of belt 10 in a timed sequence, so that the developed toner powder image contacts the advancing sheet of support material at transfer station D.
Transfer station D includes a corona generating device 60 which sprays ions of a suitable polarity onto the backside of sheet 58. This attracts the charged toner powder images from the belt 10 to sheet 58. After transfer, the sheet continues to move in the direction of arrow 62 onto a conveyor (not shown) which advances the sheet to fusing station E.
Fusing station E includes a fuser assembly, indicated generally by the reference numeral 64, which permanently affixes the transferred powder image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 and backup roller 68, with the toner powder image contacting fuser roller 66. In this manner, the toner powder image is permanently affixed to sheet 58. After fusing, a chute, not shown, guides the advancing sheet 58 to a catch tray (also not shown), for subsequent removal from the printing machine by the operator.
After the sheet of support material is separated from the photoconductive surface of belt 10, the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station F. A magnetic brush cleaner housing is disposed at the cleaner station F. The cleaner apparatus comprises a conventional magnetic brush roll structure for causing carrier particles in the cleaner housing to form a brush-like orientation relative to the roll structure and the charge retentive surface. It also includes a pair of detoning rolls for removing the residual toner from the brush.
Subsequent to cleaning, a discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining, prior to the charging thereof, for the successive imaging cycle. Stabilization of the white or background discharge voltage level is accomplished by monitoring photoreceptor white discharge level in the inter-document area of the photoreceptor using an electrostatic voltmeter (ESV) 70. The information obtained thereby is utilized by control logic 72 to control the output of ROS unit 25 so as to maintain the white discharge level at a predetermined level. Further details of this stabilization technique are set forth in U.S. Pat. No. 4,990,955, assigned to the same assignee as the present invention.
There are several scanning techniques known in the prior art to obtain the tri-level exposure imaging. A conventional flying spot scanner, such as used in the Canon 9030 uses a ROS unit to "write" an exposed image on a photoreceptive surface a pixel at a time. To obtain higher spatial resolution, a pulse imaging scanner can be utilized. This pulse imaging scanner is also referred to as a Scophony scanner in an article in Optical Engineering, Vol. 24, No. 1, Jan./Feb. 1985, pages 93 to 100, "Scophony Spatial Light Modulator", by Richard Johnson et al., whose contents are hereby incorporated by reference. A preferred technique, capable of higher spatial resolution is to use similar optical elements as the flying spot scanner (rotating polygon, laser light source, pre polygon and post polygon optics), but with an A/O modulator which illuminates many pixels at a given time, resulting in a scanner with a coherent imaging response. With this type of scan system, the exposure level, or levels at the image surface, can be controlled by controlling the drive level of the A/O modulator dependent on the video data. In a tri-level system, two drive levels are used, one for the white exposure and a second higher drive level for the DAD or color exposure.
In previous Scophony scanners, such as those disclosed in the above cited Optical Engineering article, the polygon facet itself functions as the spatial filter. However, a tri-level xerographic system is extremely power sensitive so the spatial filter within the raster output scanner must be more sensitive than merely the polygon facet.
The polygon facet also present problems with start of scan and end of scan spatial filtering while the polygon facet is spinning. A stationary spatial filter is preferable. The polygon facet as spatial filter does not provide a variable, adjustable filter. The polygon facet as spatial filter also does not permit adjusting of the frequency band that is to be reflected. The polygon facet as spatial filter does not correct for flare.
Overshoot, in the context of a tri-level xerographic system, is the unwanted power intensity above the intermediate level (all on white). In a highlight color tri-level xerographic system, black is the off level, white is the intermediate level, and color is the highest level.
Overshoot can cause print defects. When the overshoot or unwanted excess power intensity is severe enough, the prints acquire unintended color toner to appear on the page.
In a pulsed image system, the contrast of the pixel produced by the raster output scanner (ROS) is directly related to the overshoot. In a pulsed image ROS, contrast is controlled by the spatial filter. The spatial filter controls the contrast by allowing certain spatial frequencies to pass while blocking off some undesired frequencies.
It is an object of this invention to provide a non-polygon facet spatial filter in a pulsed imaging, pulse width modulation Raster Output Scanner for creating tri-level exposures to control overshoot and maintain precise intensity levels.